The present invention relates to an improved asphalt cement for use with aggregate in a paving composition.
Aggregate-containing asphalt has been employed as the paving composition for roads or the like for many years. The asphalt includes bitumen as a predominant constituent and is conventionally obtained as a solid residue from the distillation of crude petroleum. The asphalt is converted to a fluid state when paving a road. One fluid formed is the suspension or emulsion of the asphalt in water. After spreading and compressing the aggregate-containing asphalt, water evaporates and the asphalt hardens into a continuous mass. Another form of asphalt employed in road construction is a cutback, i.e., a liquid petroleum product produced by fluxing an asphaltic base with a suitable organic solvent or distillate. A road is formed by paving the aggregate-containing cutback and evaporating the volatile distillate from the mass. An advantage of using the above road construction techniques is the avoidance of high temperature application. In an alternative technique, the asphalt and aggregate can be mixed and applied at elevated temperatures at the fluid state of the asphalt to form the road. This form of asphalt, which is neither cut-back nor emulsified is referred to as asphalt cement.
A major problem with cutbacks and emulsions is their low adhesivity for aggregate in comparison to asphalt cement. This is due primarily to (a) the organic solvent or oil in the cutback and/or (b) the water in the emulsion on the aggregate surface which interferes with the formation of an adhesive bond with the aggregate and asphalt. One technique which has been disclosed to increase such adhesivity is set forth in Rogers et al U.S. Pat. No. 3,243,311. There, the aggregate is pretreated wih one of a variety of metal compounds stated to be cross-linking agents for the organic binder to oxidize polymerize or catalyze and thereby harden the binder. The pretreatment is stated to improve adhesivity of the binder and aggregate, specifically for clay-type soil aggregate. The cross-linking agents are stated to be multioxidation state metals in their higher oxidation state. The anions are stated to include a large variety of organic and inorganic acids. In addition, salts such as the halides and a large variety of inorganic oxides are mentioned. The anions are stated to include Group I, Group IV, Group V, Group VII, and Group VIII metals as well as rare earth metals. Specific examples include Cu(OH).sub.2, CuCl.sub.2, FeCl.sub.3, CuSO.sub.4, and KMnO.sub.4. In each instance, the soil is pretreated with the cross-linking agent prior to mixing with the asphalt cutback.
In Willis U.S. Pat. No. 1,328,310, an asphaltic pavement is disclosed in which copper sulfate is added to the asphalt for improving physical properties. Other compounds mentioned for this purpose include the sulfates or selenates of aluminum, chromium, iron, indium, galium, and the sulfates or selenides of sodium, potassium, rhubidium, ammonium, silver, gold, platinum or thalium. These compounds are relatively insoluble in the asphalt.
Alexander U.S. Pat. No. 2,773,777 discloses a bituminous composition particularly suitable for airport runways exposed to the high temperatures of the exhaust gases of jet engines. The composition includes bitumen emulsion, Portland cement, and mineral aggregate. To this mixture is added an aqueous solution of one of a number of water soluble salts for the purpose of giving plasticity to the composition. The salts are stated to be water-soluble polyvalent metal salts of a strong mineral acid, especially sulfuric, hydrochloric and/or phosphoric acids. The most effective salts are stated to be alkali earth metal salts including calcium chloride, magnesium chloride, barium chloride and the like. Salts of amphoteric metals are stated to be also useful including aluminum sulfate, chromium chloride and aluminum chloride. Other disclosed salts include antimony chloride, cobalt chloride, ferric chloride, antimony sulfate, cadmium sulfate and magnesium chloride. The specific examples include as salts calcium chloride, aluminum sulfate and magnesium chloride.